Ported _imp.reload() (Python/import.c) to Lib/imp.py.  Included is the change to PyImport_ReloadModule() to make it simply a wrapper around the pure Python imp.reload().  There's a good chance I don't have this right or that I have some reference leak.  I haven't worked a ton on the C side of Python (sounds tropical).  This patch also removes 'modules_reloading' from the interpreter state, since it's no longer used anywhere (see issue14618).